Summer temperatures largely control annual methane budgets of a northern Peatland

• Long-term eddy-covariance time series reveals CH 4 controls in a northern fen. • Summer CH 4 emissions dominated the fen’s annual budget, driven by soil temperatures. • Significant CH 4 releases during spring snowmelt influenced the annual budget. • Despite rising air temperatures in the region, CH₄ emissions did not increase. • This was likely due to high water tables buffering soil temperatures. Northern peatlands are a major natural source of methane (CH 4 ) worldwide. Rising temperatures due to global warming may substantially increase CH 4 emissions. Long-term studies to evaluate the impact of global warming on northern peatland CH 4 dynamics are limited, but crucial to understand the underlying controls. Here, we monitored CH 4 fluxes at a subarctic/boreal fen in northern Finland throughout 13 years (2007–2019) using the eddy-covariance technique, accompanied by measurements of abiotic and biotic drivers. Mean annual CH 4 emissions were 21.7 g CH 4 m -2 y -1 with the lowest CH 4 emissions (17.3 g CH 4 m -2 y -1 ) occurring in 2015, a relatively cold summer (1.8 °C below 2007–2019 mean), and the highest emissions (26.9 g CH 4 m -2 y -1 ) in 2018, an exceptionally warm summer (3.6 °C above 2007–2019 mean). Large CH 4 emission bursts took place during snowmelt contributing up to 7% of the annual CH 4 budget. Winter CH 4 emissions accounted for 10% to 22% of the annual CH 4 budget. Yet, summer fluxes largely controlled the annual CH 4 budget (36–47%), driven by warm soil temperatures during this season ( p < 0.001) as CH 4 emissions increased exponentially with rising soil temperatures ( p < 0.001). Observed warming of air temperatures in late summer (2.1 °C above 2007–2019 mean) did not increase CH 4 emissions as soil temperatures remained unchanged, likely buffered by high water tables. Our results suggest that, with ongoing global warming and rising summer soil temperatures, CH 4 emissions from boreal fens will likely accelerate, further amplifying global warming.